During the manufacture of flat sheet products, such as paper, plastic films, textiles, and the like, incandescent or blackbody light sources and corresponding sensors are used in measurement systems to measure constituents of a quickly moving, fluttering product web. For use with a web-manufacturing machine, measurement systems are often mounted on measurement platforms that support the light sources on one side of the web and the sensors on the other side of the web. Such sensors can also be configured in a reflection mode rather than a transmission mode. In reflection mode sensors, light sources and sensors are mounted on the same side of the web and may be mounted in a common housing. In any event, the measurement systems are scanned in a cross-process direction as the web moves relatively rapidly in a process direction. The scanning platforms are arranged so that they can move the light sources and sensors “off-sheet”, i.e., beyond at least one edge of the web product, so that an air gap is located between or adjacent to the light sources and sensors. While reflection sensors can be calibrated on-sheet, the light sources and sensors of both transmissive and reflective devices can be tested, serviced and calibrated while they are off-sheet.
Spectrographic sensors (also known as spectroscopic sensors) are common measuring devices used for measurement systems associated with web-manufacturing machines. In these applications, spectrographic sensors measure radiation transmitted through a web at specific wavelengths or wavelength bands with measurements indicating the presence or absence of different materials and, if present, the amount of the materials within the web. Exemplary materials that the sensors may measure include water, polymers, cellulose and other components of the web. A common application is the measurement of the fraction of water by weight (percent moisture) in a moving paper web during manufacturing.
Conventionally, a sensor is initially standardized with nothing between the light source and sensor so that variations in the source and measurement system can be accommodated. Then, to calibrate, test and/or verify that an infrared spectrographic sensor is performing properly, a standard physical sample, having stable, known properties, is inserted in place of the web product being manufactured. The sensor readings with the standard sample are recorded and compared with the known value of those properties of the standard sample. The sensor parameters or calibration factors conventionally stored in look-up tables or other memory devices are adjusted until the sensor readings conform to the known values of the standard sample. To provide automated testing and/or verification, a standard sample may be internal to the measurement system and automatically inserted in place of the web, increasing the complexity of the measurement system. With an internal standard sample in place, a sensor measurement is taken and a check or automated calibration is performed for the sensor.
Unfortunately, the construction and maintenance of standard samples can be problematic. Initially, the standard sample must adequately simulate the properties of the web product that the sensor is measuring and the properties of the standard sample that serve as the basis for tests and/or calibrations must be extremely stable. Once such a sample is prepared, it must be able to remain stable and not be destroyed or damaged while being used and stored in the full range of harsh manufacturing environments that web-manufacturing may encounter including, for example, high humidity, dirt, heat and water in the case of paper manufacturing. Further, the sample must be able to withstand exposure to the light source itself which, when concentrated for testing, standardization and/or calibration, may be intense and tends to alter the properties of the sample. To meet these requirements, the standard sample may need to be protected for example by placement in a special storage environment between uses.
Further, if the standard sample becomes lost, contaminated, aged, damaged or otherwise unavailable, it may be expensive and time consuming to replace the standard sample. And, until the standard sample is replaced, the sensor may not be able to be calibrated or have readings verified except by laborious preparation of short-lived samples at the web-manufacturing facility.
Accordingly, there is a need for a calibration and verification reference sample for a measurement system including a blackbody radiation source and a spectrographic measuring device that overcomes or lessens these problems.